Eventually, she was able to escape Kavanaugh's grasp, but the memory had been a source of stress ever since, she wrote in the letter.

A classmate of Kavanaugh was allegedly present during the incident, but he said he "had no recollection of that".

The letter was written to Senator Diane Feinstein, ranking minority member of the Senate Judiciary Committee, last July, shortly after US President Donald Trump announced he would nominate Kavanaugh for the Supreme Court.

Feinstein has since referred the letter to the Federal Bureau of Investigation (FBI).

A statement put out by the committee chairman Senator Chuck Grassley's office said the allegations will not hinder the committee vote on Kavanaugh, which will take place on Thursday.

"Judge Kavanaugh has undergone six FBI full-field investigations from 1993 to 2018," Grassley said in a statement.

"No such allegation resembling the anonymous claims ever surfaced in any of those six FBI reports."

Kavanaugh is expected to be confirmed by the one-vote Republican Senate majority, despite Democrats attempts to stop his confirmation.

Kavanaugh's appointment could solidify a hard-right majority on the nine-member court and help shape key aspects of American society for a generation or more.

Conservative Catholic Kavanaugh, who hails from a wealthy suburb of Washington, DC, has the backing of powerful right-wing judicial groups and deeply religious evangelical Christians.

Both groups are pro-gun and anti-abortion rights and form a key part of the Republican voter base.

Other issues to come into play during the hearings are his support for corporations against regulation, and the judge's belief that a sitting president should not be distracted by legal proceedings against him.

Kavanaugh's thoughts on the last issue are especially important as Trump was alleged to have taken part in late 2016 in making payments to an adult film star and a Playboy model in possible violations of election laws.

In recent weeks, Democrats have been fiercely critical of the Trump administration for not providing sufficient documentation about Kavanaugh's time in Bush's White House, where he may have played key roles in decisions such as permitting the torture of detainees.